The first electrical measurements of atomically thin layers of graphene initiated a flurry of research activity to explore and exploit the superlative properties of two-dimensional nanomaterials. Graphene, the archetypal 2d material, can provide electronic mobilities in excess of 200,000 cm2 V−1 s−1, and exhibits the highest strength of any known material. These properties and others have been harnessed in a range of different applications, including supercapacitors, transparent conductors, wideband photodetectors, and RF electronics. Despite these developments, the applications of graphene have been fundamentally limited by zero-bandgap semiconducting or semi-metal behavior, which prevents its incorporation into conventional field-effect transistor devices. Researchers have thus turned to other elements in the periodic table to unearth new 2d materials with beneficial new properties. Transition metal dichalcogenides such as MoS2, MoSe2, and WS2, are 2d semiconductors that provide direct bandgaps and strong spin-orbit effects, making them promising materials for optoelectronic and spintronic devices. Boron nitride (an electrical insulator with unusually high thermal conductivity) has been used as a high-performance gate dielectric for electronic devices, and in flexible, high-temperature dielectric nanocomposites.
Despite considerable research activity focused on similar layered materials, the metal diborides remain a largely unexplored class of potential 2d nanomaterials. These compounds have the general formula MB2, where M is metal such as Mg, Al, or Ti. They consist of layers of boron arranged in a honeycomb lattice that are separated by metal atoms centered atop each boron hexagon. Their structure is thus similar to that of intercalated graphite. Although the metal diborides share the same structure, they possess a diverse range of materials properties. MgB2 is a well-known superconductor with a 39K transition temperature, which ranks it among the highest of the conventional superconductors. AlB2 and boron-enriched NbB2+x are also superconducting. Transition metal diborides, such as ZrB2, HfB2, and TiB2, are highly refractory with melting temperatures above 3000° C., and provide high mechanical hardness, strong resistance to chemical attack and erosion, and high electrical conductivities. These ultrahigh temperature ceramics (“UHTCs”) are ideal materials for use in extreme conditions, such as in coatings for cutting tools, high temperature electrodes, and components for hypersonic flight and atmospheric reentry vehicles. TiB2, in particular, also possesses low density and low Poisson's ratio for use in armor and exhibits electrical conductivity up to 1.6×107 S m−1 (at 20° C.), which is higher than that of elemental Ti and within a factor of four of copper. Due to their extremely high melting temperatures, efficient means of sintering the transition metal diborides remains a key challenge preventing their more widespread adoption.
The metal diborides, with their graphene-like boron sheets, are also related to boron nanostructures garnering increasing research interest. Boron allotropes have long been viewed as potential companion compounds to the well-known carbon nanoscale allotropes buckminsterfullerene (C60), carbon nanotubes, and graphene. Like carbon, boron is well known for its ability to form covalent bonds with diverse elements and form molecular networks. Accordingly, there have been many attempts to form boron equivalents of the archetypal carbon nanomaterials. Theoretical treatments have predicted that these boron compounds could provide novel electronic properties and in some cases, electrical conductivity that exceeds that of carbon nanotubes. Unfortunately, it has remained challenging to synthesize boron nanostructures since they have proved to be less thermodynamically stable than more common allotropes. Researchers have been making steady progress synthesizing all boron nanostructures, including clusters, single-walled boron nanotubes, and most recently borosphorene (B40), the boron equivalent of C60. Synthesis of the borophene, the boron equivalent of graphene, remains an outstanding challenge in materials science and chemistry. Moreover, dispersing metal diborides also remains a challenge. Current techniques to disperse these compounds have required ion intercalation (see for example T. T. Salguero, C. A. Barrett & D. Sexton. Nanoparticles And Method Of Making Nanoparticles. 20150140331 (2015)), or hydroxylation (see for example S. K. Das, A. Bedar, A. Kalman & K. Jasuja, “Aqueous dispersions of few-layer-thick chemically modified magnesium diboride nanosheets by ultrasonication assisted exfoliation,” Scientific Reports 5 (2015).). These techniques tend to produce suspensions that yield compounds with starkly different chemical properties from pristine metal diborides.